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BOURNONITE

THE MINERAL BOURNONITE

Bournonite is also called "cog wheel ore" because of the cog wheel shape that the twinned crystals form. Twinning is common in bournonite and if a crystal repeatitively twins it can form a type of twin called a trilling. The trilling is composed of four "twins" or crystals connected in a plane and forms a wheel with a jagged edge that resemble the teeth of a cog wheel. Not all specimens show this type of twinning however.

PHYSICAL CHARACTERISTICS:

Color is silver gray or black.

Luster is metallic.

Transparency crystals are opaque.

Crystal System is orthorhombic; 2/m2/m2/m

Crystal Habits include tabular to prismatic crystals. Twinning is common and if repeated forms flat wheel shaped crystals called cog wheels. Also massive and granular.

Cleavage is poor in one direction.

Fracture is subconchoidal.

Hardness is 2.5 – 3

Specific Gravity is approximately 5.8 (above average for metallic minerals)